1. Field of the Invention
This invention relates to a holographic color image reconstructing device, and more particularly to a device for reconstructing a bright color image hologram by use of a white light source.
2. Description of the Prior Art
In holography, color or non-color, it is a great desire to increase the diffraction efficiency that is the rate at which the energy of the light beam contributing to reconstruction of images occupies the total energy of the reconstruction light beam. Owing to the low diffraction efficiency, the field of application of holography has been materially limited. Particularly, it has been considered almost impossible to apply the holographic techniques to the field of audio-visual education wherein a number of people observe a single large projection screen on which holographic images are reconstructed.
In order to enhance the diffraction efficiency in holography, it has been proposed to use a phase-only hologram which records holographic information by use of phase variation of the recording media. For phase-only materials, the required phase shift of the reconstruction wave is provided through the local variations in the thickness and/or the refractive index of the recording medium. When photograhic emulsions which are absorptive are employed for the phase-only materials, developed photographic emulsions are bleached in a manner such that a phase-only image will remain. The diffraction efficiency when photographic emulsions are used, however, is very low. For instance, even when information of a single point is recorded and reconstructed, the diffraction efficiency is about 30% at highest. When a general object is recorded and reconstructed the diffraction efficiency is as low as several percent. Further, the dichromate sensitized gelatin, lithium niobate or photosensitive resist are known to be used as phase-only materials. However, gelatin dichromate cannot be practically used due to its short life, lithium niobate cannot be used either due to its very low sensitivity and the photosensitive resist is disadvantageous in that the range of the sensitive wavelength is limited between 200 and 500 m.mu..
Therefore, from the viewpoint of the characteristics of various recording materials, there is a limit in the diffraction efficiency in holography. Accordingly, it has been desired to enhance the brightness of the holographically reconstructed image without decreasing the diffraction efficiency.
One method of increasing the brightness without decreasing the diffraction efficiency is to use a laser beam source of high power. However, the high power laser beam source is economically disadvantageous and occupies a large space. Particularly, in case of reconstructing color image holograms, it is practically impossible to use such high power laser beam sources of several colors.
In view of the above considerations, it has been desirable to use ordinary nonexpensive white light sources such as halogen lamps, mercury lamps, xenon lamps, etc. instead of multi-color laser beam sources as light sources for reconstruction of color hologram images. A color hologram from which a multi-color image can be reconstructed is known as a Lipmann type color hologram. The Lipmann type color hologram, however, is disadvantageous in that the diffraction efficiency is low and the duplication thereof is difficult. Further, since the interference pattern in the Lipmann type color hologram is recorded in parallel to the surface of the recording material, the photographic emulsion is shrunk after development and fixing processes which results in color shift in the multi-color reconstruction image by varying the spaces between stripes of the interference pattern.